The items in a ResourceDictionary are not immediately processed when application code is loaded by a XAML loader. Instead, the ResourceDictionary persists as an object, and the individual values are processed only when they are specifically requested.

In Extensible Application Markup Language (XAML), the ResourceDictionary class is typically an implicit collection element that is the object element value of several Resources properties, when given in property element syntax. For details on implicit collections in XAML, see XAML Syntax In Detail. An exception is when you want to specify a merged dictionary; for details, see Merged Resource Dictionaries.

Another possible XAML usage is to declare a resource dictionary as a discrete XAML file, and either load it at run time with Load or include it in a (full-trust) project as a resource or loose file. In this case, ResourceDictionary can be declared as an object element, serving as the root element of the XAML. You must map the appropriate XML namespace values (default for the WPF namespace and typically x: for the XAML namespace) onto the ResourceDictionary element if you plan to use it as the root element. Then you can add child elements that define the resources, each with an value.